● The Table gives a comparison of the different transport mechanisms.

● `color{violet}("Proteins in the membrane")` are responsible for `color{brown}("facilitated diffusion")` and `color{brown}("active transport")` and hence show common characterstics of

`star` Being highly `color{brown}("selective; ")`
`star` They are liable to `color{brown}("saturate,")`
`star` Respond to `color{brown}("inhibitors")` and
`star` Are under `color{brown}("hormonal")` regulation.

● But `color{violet}("diffusion whether facilitated")` or not – take place only `color{brown}("along a gradient")` and do `color{brown}("not use energy")`

● `color{brown}("Water")` is essential for all `color{violet}("physiological activities")` of the plant and plays a very important role in `color{violet}("all living organisms.")`

● It provides the medium in which most substances are `color{violet}("dissolved.")`

● The `color{brown}("protoplasm")` of the cells is nothing but water in which `color{violet}("different molecules")` are dissolved and (several particles) suspended.

● A `color{brown}("watermelon")` has over 92 per cent water; most `color{brown}("herbaceous plants")` have only about 10 to 15 per cent of its fresh weight as `color{violet}("dry matter.")`

● Of course, distribution of water within a plant varies – `color{brown}("woody parts")` have relatively very little water, while soft parts mostly contain water.

● A `color{brown}("seed")` may appear dry but it still has water – otherwise it would not be alive and respiring!

● `color{violet}("Terrestrial plants")` take up huge amount water daily but most of it is lost to the air through evaporation from the leaves, i.e., `color{brown}("transpiration.")`

● A mature corn plant absorbs almost `color{brown}("three litres")` of water in a day, while a mustard plant absorbs water equal to its `color{brown}("own weight")` in about 5 hours.

● Because of this high demand for water, it is not surprising that water is often the `color{brown}("limiting factor")` for plant growth and productivity in both agricultural and `color{violet}("natural environments.")`

● To `color{violet}("comprehend plant-water relations")`, an understanding of certain standard terms is necessary.

● `color{brown}("Water potential" (ψ_w))` is a concept fundamental to understanding `color{violet}("water movement.")`

● `color{brown}("Solute potential" (ψ_s))` and `color{brown}("pressure potential" (ψ_p))` are the two main components that determine `color{violet}("water potential.")`

● Water molecules possess `color{brown}("kinetic energy.")`

● In `color{violet}("liquid and gaseous")` form they are in random motion that is both rapid and constant.

● The greater the `color{brown}("concentration")` of water in a system, the greater is its `color{violet}("kinetic energy or ‘water potential’.")`

● Hence, it is obvious that `color{brown}("pure water")` will have the greatest `color{violet}("water potential.")`

● If two systems containing water are in contact, random movement of water molecules will result in net movement of water molecules from the system with higher energy to the one with lower energy.

● Thus water will move from the system containing water at higher water potential to the one having low water potential.

● This process of movement of substances down a gradient of free energy is called `color{brown}("diffusion.")`

● `color{violet}("Water potential")` is denoted by the Greek symbol `color{brown}("Psi" "or" ψ )`and is expressed in pressure units such as `color{brown}("pascals (Pa).")`

● By convention, the water potential of pure water at standard temperatures, which is not under any pressure,
is taken to be `color{brown}("zero.")`

● If some solute is dissolved in `color{violet}("pure water,")` the solution has fewer free water and the concentration of water decreases, reducing its `color{violet}("water potential.")`

● Hence, all solutions have a lower water potential than `color{violet}("pure water;")` the magnitude of this lowering due to dissolution of a solute is called `color{brown}("solute potential")` or `color{brown}(ψ_s. ψ_s)` is always negative.

● The more the`color{brown}(" solute molecules")`, the lower (more negative) is the s .

● For a solution at atmospheric pressure (water potential) `ψ_w` = (solute potential) `ψ_s`.

● If a pressure greater than atmospheric pressure is applied to `color{violet}(" pure water")` or a solution, its `color{violet}("water potential")` increases.

● It is equivalent to `color{violet}("pumping water ")` from one place to another.

● Pressure can build up in a `color{violet}("plant system")` when water enters a plant cell due to `color{violet}("diffusion causing ")` a pressure built up against the cell wall, it makes the cell `color{brown}("turgid ")` this increases the `color{brown}("pressure potential.")`

● Pressure potential is usually positive, though in plants `color{brown}("negative potential or tension")` in the water column
in the `color{violet}("xylem plays")` a major role in water transport up a stem.

● `color{brown}("Pressure potential")` is denoted as `Psi_p.`

● `color{violet}("Water potential of a cell")` is affected by both solute and pressure potential. The relationship between them is as follows:
`Psi_w = Psi_s + Psi_p`